R.M. Dawson

Electron‐transport mechanisms in metal Schottky barrier contacts to hydrogenated amorphous silicon

Detailed electrical studies have been carried out on nickel, palladium, and platinum Schottky barrier contacts to intrinsic a‐Si:H films of 0.5, 1.0, and 3.0 μm thickness. The diode characteristics of the Schottky barrier structures are investigated using light and dark I‐V measurements over a temperature range of 24–150 °C. The activation energies of the diode currents are compared to the electron barrier heights determined from internal photoemission (IPE) measurement. The bias dependence of the reverse diode currents is directly compared to the barrier lowering measured by IPE. The diode currents are explained in terms of a joint thermionic‐emission/drift‐diffusion model that includes changes in the barrier height with applied bias.

Charge-defect thermodynamic equilibrium and "metastable" defects in amorphous silicon

A thermodynamic equilibrium description of the high‐temperature (150–250 °C) steady‐state behavior of light‐induced defects in amorphous silicon is presented. The entropy and enthalpy of dangling‐bond formation are quantified. In contrast to the behavior of vacancies in single‐crystalline silicon the creation of the dangling‐bond defect in amorphous silicon produces negative entropy and enthalpy changes indicating that lattice relaxations contribute to the free‐energy changes. Over the temperature range examined, the creation of dangling bonds lowers the free energy due to the relatively large negative enthalpy change. Practical issues such as the estimation of the saturated dangling‐bond density resulting from given illumination level at temperatures too low to experimentally observe true saturation are also considered.

Effects of microstructure on transport properties of undoped hydrogenated amorphous silicon films

Electronic transport properties have been investigated in undoped hydrogenated amorphous silicon (a‐Si:H) materials whose microstructure and void fraction are changed by deposition temperature (Ts). The hydrogen content in these materials decreases from 15 to 5 at. % and the void fraction by 14% as Ts is raised from 200 to 350 °C. The photo and dark conductivities are measured from 40 to 190 °C and extended state electron mobilities are derived from a self‐consistent analysis. The room temperature mobilities are found to increase from 0.8 to 30 cm2/V s and become less temperature dependent as Ts increases. These temperature activated mobilities explain the Meyer–Neldel rule [Z. Tech. Phys. 18, 588 (1937)] in a‐Si:H materials whose dark conductivity activation energies are greater than 0.4 eV where it cannot be explained by the statistical shift of the Fermi level.

Charge-defect equilibrium description of metastable defect concentrations

A thermodynamic equilibrium framework in which charge carriers are in equilibrium with charged and uncharged dangling bonds (DB) is developed. Thermal and light DB creation are related. DB formation is an exothermic reaction with negative entropy and free energy changes. Thus, the lowest energy state is with the weak bonds split into DB!

Photocarrier Transport and Recombination in Amorphous Silicon

The effect of microstructure in undoped a-Si:H films on carrier transport, recombination, densities of midgap states and solar cell characteristics has been investigated. Extended state mobilities of electrons were obtained from photo and dark conductivity measurements between 40° C and 190° C and the gap states characterized using Dual Beam Photoconductivity. In these films the estimated room temperature electron mobilities increase from about 1 to 30 cm 2 /V sec as the dihydride concentrations and void volume fractions decrease. It is found that the carrier mobility-lifetime products are not solely determined by the dangling bond states. The effects of changes in the mobilities and midgap states on p-i-n homojunction solar cell characteristics are presented and discussed.

Optoelectronic Properties of Plasma CVD a-Si:H Modified by Filament-Generated Atomic H

We have studied a-Si:H prepared by alternating plasma deposition with atomic H treatments performed with a heated W filament. Real time spectroscopie ellipsometry provides the evolution of film thickness, optical gap, and a measure of the fraction of Si-Si bonds broken in the near-surface (200 A) during H-exposure of single films. This information guided us to the desired parameters for the H-treatments. Here, we concentrate on a weak hydrogenation regime characterized by minimal etching, a higher H content by 2 at.%, and a larger optical gap by 0.02 eV for the growth/hydrogenation structures in comparison to continuously deposited control samples. This new material has shown an improvement in the defect density in the light-soaked state in comparison to the control samples. This may result from stabilization of the Si structure due to an increase in the H chemical potential in the a-Si:H.

Amorphous silicon dispersive transport considerations for analysis of films and solar cells

Abstract It has been reported that as the deposition temperature is decreased, the hydrogen content increases, the microvoid fraction increases and the electron mobility decreases. Based on dispersive transport considerations, it is not expected that the electron mobility plays a major role in determining μ e τ e or affects the performance of solar cells in the annealed state. However, if a thermodynamic equilibrium exists between charge carriers and dangling bond defects, lower mobilities will have a detrimental affect on stability due to increases in carrier density. Furthermore, the distributions of charged dangling bonds are affected by the disparity between electron and hole mobilities which, under illumination, will result in asymmetrical distributions of charged, light induced defects.

The Staebler-Wronski Effect and the Thermal Equilibration of Defect and Carrier Concentrations

Light induced degradation of intrinsic Amorphous silicon (a-Si:H) is investigated as a function of temperature. Previous work described an equilibrium framework for the high temperature behavior of dangling bonds defects (DB) 11]; and the temperature dependence of the annealed state photo, σ PH , and dark, σ D , conductivities of a series of intrinsic a-Si:H Materials deposited over a range of substrate temperatures, 200°C s D and σ PH , decrease compared to the annealed state while the ratio, σ D /σ PH remains unchanged. Relationships between the ratio [DB + ]/[DB] and the Fermi level are derived from the equilibrium framework.

Investigation of intrinsic defect states in hydrogenated amorphous silicon films using steady-state photoconductivity and sub-bandgap absorption

The density, distribution and nature of intrinsic gap states in a-Si:H films deposited over a wide range of substrate temperatures have been investigated using steady-state photoconductivities and sub-bandgap absorption. The sub-bandgap absorption was obtained using dual beam photoconductivity (DBP) measurements and the results were analyzed using a detailed numerical model based on Rose-Simmons-Taylor statistics. This model takes into account the effects of mobility, recombination velocity, position of Fermi level, and the presence of sensitizing states. It is found that there is a large effect of sensitizing states and position of Fermi level on both electron mobility-lifetime products and sub-bandgap photoconductivities used to obtain information about gap states in different films.<<ETX>>

Transport considerations in hydrogenated amorphous silicon materials with widely varying mobilities and the consequences on device performance

The transport properties of device quality hydrogenated amorphous silicon (a-Si:H) materials deposited at different substrate temperatures, T/sub s/, are investigated. The mobilities are found to vary continuously from about 1 cm/sup 2Vsec to 30 cm/sup 2Vsec as T/sub s/ increases from 220/spl deg/C to 350/spl deg/C and, in some cases, the mobilities are temperature activated. These results provide an explanation for the Meyer-Neldel rule in a-Si:H films with dark conductivity activation energies greater than 0.4 eV where it cannot be explained by the statistical shift of the Fermi level. The solar cells formed with the different intrinsic layers exhibit decreasing short-circuit currents, open circuit voltages and fill factors despite the increased mobilities with T/sub s/. These results need to be carefully modeled in order to obtain further insights into the operation of a-Si:H p-i-n solar cell operation.<<ETX>>